The present invention relates generally to passive optical devices for channel selection, and more particularly, to devices in which dielectric thin-film filters are used for channel selection.
There is currently a market need, experienced by, for example, telephone carriers and cable operators, to provide short-haul transmission of internet protocol (IP) packets among interconnected nodes of a network that are typically spaced apart by 20 km or less. Proposed systems send IP packets, via a SONET interface, over a fiber-optic transmission medium using wavelength division multiplexing (WDM). A particular wavelength channel is assigned to each node. At each node, optical demultiplexing is required to extract the received data from the medium in the channel of interest, and optical multiplexing is required to inject the transmitted data into the medium in the channel of interest.
The optical components that effectuate optical multiplexing and demultiplexing add cost to the network installation, and in operation, they also add optical losses that accumulate over the network. For greater profitability, it is desirable both to reduce installation cost, and to reduce the accumulated losses that would otherwise limit transmission distances. For both purposes, it would be advantageous to reduce the number of optical components at each node that perform optical multiplex-demultiplex functions.
One known device for providing the multiplex-demultiplex function described above is an optical add-drop module (OADM) using in-line dielectric thin-film filters (TFFs). Such a device is represented in FIG. 1. Very briefly, incoming traffic on optical fiber 10 is incident on drop filter 15. Signals in undesired wavelength channels, i.e. all but the channel assigned to the node of interest, are reflected into bypass optical fiber 20. From fiber 20, these signals are incident on add filter 25 and reflected into optical fiber 30, from which they propagate downstream and away from the node of interest. Signals in the channel of interest are transmitted through filter 15 to receiver 35. Signals in the channel of interest from transmitter 40 are transmitted through filter 25 into optical fiber 30.
Some incoming optical power in the channel of interest may leak through the OADM by reflection from filters 15 and 25 and transmission into fiber 30. Such leakage is undesirable because it leads to interference in that channel. The attenuation of the undesired power, relative to the desirably transmitted power in the channel, is referred to herein as channel isolation. Those skilled in the art will appreciate that the term adjacent channel isolation is often used to denote a measure of the suppression of channel crosstalk at receiver 35. What we mean by channel isolation, however, is the isolation provided by virtue of the reflections from filters 15 and 25. Such isolation is also sometimes referred to as reflection isolation.
In some cases, the add and drop filters fail to provide sufficient channel isolation. In those cases, it is customary to add a third filter 45, which supplements filters 15 and 25 by continuing the bypass path from fiber 20 to fiber 20xe2x80x2. The additional reflection from this third filter further reduces the power in the local add/drop channel that is transmitted into fiber 30, and thus further increases the channel isolation.
There are certain drawbacks to the use of a third filter, such as filter 45, in the bypass fiber. Such a filter adds some loss to all of the traffic being routed through the bypass fiber, and it adds component cost to the fiber-optic network.
Represented in FIG. 2 is a pair of characteristic curves for an illustrative TFF. Curve 50 is a transmission characteristic, and curve 55 is a reflection characteristic. Evident in curve 55 are ripple features 60 having peaks 61 and valleys 62. A ripple peak corresponds to a wavelength, within the transmission passband of the TFF, which is also partially reflected. Thus, the TFF affords somewhat less isolation near ripple peaks than it does near ripple valleys. Herein, we use the term across-the-band isolation to refer to the smallest isolation achieved at any point within the passband of a given TFF.
We have found that the combined isolation afforded in an OADM by an add filter and a drop filter can be improved by selecting a pair of filters having nominally similar characteristics, but having relatively displaced ripple features, such that at least one ripple peak of the add filter falls at approximately the same wavelength as a ripple valley of the drop filter and vice versa. The resulting improvement is sufficent to obviate the need for a third filter in the bypass fiber, even for some highly demanding applications. Thus, appropriate matching of the add and drop filters can lead to reduced loss in the network, and reduced network cost.
Accordingly, the invention in one embodiment is an OADM having a passband and a through-path, and in the throughpath, exactly two broadband TFFs having nominally similar transmission and reflection characteristics. In this context, the throughpath is the path taken by traffic that is not added or dropped. With reference to FIG. 1, it is the path from fiber 10 to fiber 30 via fibers 20 and 20xe2x80x2. The respective TFFs are selected to have at least partially complementary reflection characteristic curves, such that at least some ripple peaks in the characteristic curve of one TFF overlap ripple valleys in that of the other TFF. The selection of TFFs is further characterized in that the OADM achieves a level of channel isolation over the entire said passband that is greater than that achievable by pairing either of the individual TFFs with an identical counterpart.